The invention is directed to addition-curable polysiloxane potting compounds used to encapsulate circuitry.
A thermally conductive material is conventionally used to establish thermal contact between the heat sink and the heat-producing components and/or the circuit board. Illustrative disclosures relating to heat dissipation in circuit boards include Cipolla et al., U.S. Pat. No. 5,268,815 (1993); Kim et al., U.S. Pat. No. 5,552,635 (1996); and Shuff, U.S. Pat. No. 5,812,374 (1998).
The present invention is directed to addition-curable silicone gels and readily adapted as pottants and encapsulants for preserving and protecting delicate electronic circuitry in severe environments. Additional applications of silicone gels according to the present invention include optical joint sealants, optical waveguides, keyboard handrests, and clean room HEPA filter sealants. Conventional addition-curable silicone gel compositions comprising an alkenyl-containing polydiorganosiloxane polymer, an organohydrogenpolysiloxane, and a hydrosilylation catalyst are known in the art from U.S. Pat. No. 4,374,967 to Brown et al.; U.S. Pat. No. 4,529,789 to Kroupa; U.S. Pat. No. 5,332,795 to Fujiki et al.
Specifically, there is provide in the art crosslinked silicone prepared by the platinum-catalyzed reaction between a vinyl-functionalized PDMS and a hydride-functionalized PDMS. Gelest, Inc. produces commercial curing packages useful with polydimethylsiloxane. Such gels can be formed in a number of ways. One method synthesizes the crosslinked polymer in the presence of a non-reactive extender fluid, e.g. trimethylsiloxy-terminated PDMS. The method according to the present invention fabricates the silicone gel by reacting a stoichiometric excess of a multifunctional vinyl-substituted silicone with a multifunctional hydride-substituted silicone in such a fashion that a soft, fluid-extended system is obtained.
In the reaction of a multifunctional vinyl-substituted silicone with a multifunctional hydride-substituted silicone, a vinyl-rich sol fraction is obtained. Suitable examples of either of these gel systems are taught in, inter alia, Debbaut, U.S. Pat. No. 4,600,261 (1986); Debbaut, U.S. Pat. No. 4,634,207 (1987); Debbaut, U.S. Pat. No. 5,357,057 (1994); Dubrow et al., U.S. Pat. No. 5,079,300 (1992); Dubrow et al., U.S. Pat. No. 4,777,063 (1988); and Nelson, U.S. Pat. No. 3,020,260 (1962); the disclosures of which are incorporated herein by reference.
U.S. Pat. No. 5,571,853 to Ikeno et al. discloses a gel-forming silicone composition comprising (A) an organopolysiloxane containing a silicon-bonded alkenyl group or groups in an average amount of from 0.15 to 0.35 mol % based on all silicon-bonded organic groups contained per molecule, (B) a non-functional organopolysiloxane, (C) an organohydrogenpolysiloxane containing an average of 2 silicon-bonded hydrogen atoms per molecule, and (D) an addition reaction catalyst. The silicone composition is able to give a gel-like cured silicone product with low modulus of elasticity while retaining flexibility.
U.S. Pat. No. 6,225,433 discloses a curable silicon composition, comprising (A) 100 parts by weight of an organopolysiloxane containing silicon-bonded aryl groups and at least two alkenyl groups per molecule, and having a viscosity of from 0.01 to 1,000 Pa-s at 25xc2x0 C., wherein the aryl groups comprise from 1 to 40 mole % of the total silicon-bonded organic groups in the organopolysiloxane; (B) an organopolysiloxane having a viscosity of from 0.001 to 10 Pa-sec. at 25xc2x0 C. and containing at least 2 silicon-bonded hydrogen atoms per molecule, in a quantity sufficient to cure the composition; (C) a platinum catalyst in a quantity sufficient to cure the composition; and (D) 0.00001 to 100 parts by weight of an organopolysiloxane having a viscosity of from 0.01 to 10,000 Pa-s at 25xc2x0 C. provided that when the organopolysiloxane contains aryl groups, the aryl groups comprise less than 1 mole % or more than 40 mole % of the total silicon-bonded organic groups in the organopolysiloxane. The organopolysiloxane cures to form a silicone having a complex modulus less than or equal to 1xc3x97108 Pa at xe2x88x9265xc2x0 C. and a shear frequency of 10 Hz or a cured silicone having a Young""s modulus less than or equal to 2.9xc3x97108 Pa at xe2x88x9265xc2x0 C.
Thus, it is desirable to develop a potting compound for encapsulating circuitry having high thermoconductivity for dissipating heat and/or transferring heat to a heat sink, and away from the circuit assembly, while at the same time protecting the circuitry from mechanical stresses or avoiding the transmission of such stresses to the circuitry. A spin castable potting compound must be able to flow under its own weight when applied to a circuit module mounted on a turntable spinning at above 1000 rpm. The viscosity of a potting compound as measured by Brookfield DV-II, spindle 7 at 10 rpm after a 5-minute dwell of less than 150,000 cps.
Exemplary of the current state of the art, thermally conductive spin castable pottants exhibit a thermal conductivity of about 0.5 w/mxc2x0K. Higher thermal conductivity is desired.
In attempts to increase the amount of thermally conductive filler loadings an unacceptable rise in viscosity occurs which would prohibit the application of the spin casting method. Improved thermal conductivity in the use of high loadings of conductive filler in a spin castable composition is desired.
In a basic aspect of the invention a thermally conductive silicone gel is produced by reacting a stoichiometric excess of a multifunctional vinyl-substituted silicone with a multifunctional hydride-substituted silicone. The thermal conductive components are a combination of at least three conductive solid fillers, aluminum oxide, zinc oxide and boron nitride.
In another aspect of the invention there is provided a high thermal conductive spin castable potting compound embodied in a two-part addition-crosslinkable composition wherein a vinyl-terminated siloxane crosslinks with Si-bonded hydrogen (hydrosilylation) in the presence of a platinum catalyst and contains a combination of thermal conductive materials comprising aluminum oxide, zinc oxide and boron nitride, in the substantial absence of aluminum nitride, as specified hereinbelow. In an alternative aluminum oxide, zinc oxide, titanium dioxide and boron nitride comprise the thermal conductive fillers.
In another aspect there is provided a method for encapsulating circuitry, comprising the steps of spin casting a precured silicone composition, said composition comprises:
a mixture of a stoichiometric excess of a multifunctional vinyl-substituted silicone with a multifunctional hydride-substituted silicone together with alumina, zinc oxide and optionally boron nitride, wherein the precured pottant exhibits a Brookfield DVII (#7 spindle) 1/10 rpm of less than 250,000/175,000 cps at 25xc2x0 C., a thermal conductivity of at least 2.75 to 4 W/mK and a Shore A hardness after curing of from 30 to 80.